Changes in lymphatic organs of layer chickens following vaccination against Marek's disease: histological and stereological analysis

The aim of this study was to investigate histomorphometrical
characteristics of the thymus, bursa of Fabricius and spleen in the
chickens vaccinated with a vaccine against Marek’s disease. For this
purpose, we used newly hatched chickens of the light hybrid line,
obtained from a local hatchery. The chickens were vaccinated on the
5th day after hatching with a bivalent cell-associated Marek's disease
vaccine (PFU-2000 per dose). On day 13 both vaccinated chickens and
unvaccinated controls were sacrificed, and thymus, bursa and spleen
were removed and processed for light microscopy. The serial tissue
sections, hematoxylin-eosine stained, were used for
histomorphometric analysis. Vaccination against Marek’s disease
decreased the relative mass of the lymphoid organs, and caused
significant damage of the thymus and spleen in experimental chicken.
In addition, vaccination, similar to Marek's disease virus, induced
morphometric changes in the lymphoid organs. Namely, it significantly
decreased the diameter and volume of lymphoid follicles, volume of
follicular medulla and number of cells in the follicular cortex in the bursa
of Fabricius. In the thymus, vaccination reduced the thymus volume and
the absolute number of thymocytes. However, vaccination against
Marek’s disease caused an increase in the diameter, number and
volume of lymphoid follicles in the spleen.
The present data suggest that vaccination against Marek’s
disease was able to induce the immune response in processed organs,
although it reduced the mass and number of lymphocytes in the major
lymphoid organs.